The present application is based on Japanese priority application No. 2000-131439 filed on Apr. 28, 2000, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to semiconductor devices and more particularly to a high-speed photodetector.
Avalanche photodiodes are used extensively in optical fiber telecommunication systems and networks for detection of optical signals. Particularly, the avalanche photodiode of the system InGaAs/InP is capable of detecting optical signals having a transmission rate of as high as 10 Gb/s and is important in optical fiber telecommunication trunk as a high-speed photodetector.
The avalanche photodiode of the InGaAs/InP system uses an InGaAs layer having a lattice matching composition with respect to an InP substrate as an optical absorption layer. The InGaAs layer having such a lattice matching composition has a photosensitivity for optical radiation at the wavelength of 1.55 xcexcm, wherein the wavelength of 1.55 xcexcm is used extensively in the art of optical fiber telecommunication in view of minimum transmission loss of optical signals transmitted therethrough.
An avalanche photodiode has an additional advantageous feature of reduced carrier transit time associated with the use of a SAM (separated absorption and multiplication) structure for the carrier multiplication layer of InP. As a result of the reduced carrier transit time, the response speed of the photodiode is improved substantially.
FIGS.1A and 1B show a cross-sectional view and a band structure of a conventional InGaAs/InP avalanche photodiode 10.
Referring to FIG.1A, an optical absorption layer 12 of n-type InGaAs is formed on a substrate 11 of n+-type InP epitaxially, and a carrier multiplication layer 13 of nxe2x88x92-type InP is provided on the optical absorption layer 12 epitaxially. Further, a biasing layer 14 of p-type InP is formed on the carrier multiplication layer 13 and a guard ring 14A of p+-type InP is formed in the biasing layer 14 so as to surround an optical path of an incident optical beam.
In operation, a reverse bias voltage is applied across the InP substrate 11 and the guard ring 14A with such a magnitude that the carrier multiplication layer 13 is biased to the point near avalanche breakdown, and excitation of electron-hole pairs is caused in the optical absorption layer 12 in response to irradiation of the incident optical beam.
The electrons thus excited are immediately absorbed by an electrode provided on the InP substrate 11, while the holes are transported toward the biasing layer 14 through the carrier multiplication layer 13 while being accelerated by the electric field induced by the reverse bias voltage. Thereby, each of the accelerated holes causes a collision with the crystal lattice of the carrier multiplication layer 13, and such a collision induces a subsidiary excitation of electron-hole pairs.
The electrons thus excited by the collision are absorbed by the electrode on the InP substrate 11, while the holes are transported through the carrier multiplication layer 13 toward the foregoing biasing layer 14 while being accelerated by the reverse bias voltage. Thus, each of the accelerated holes undergoes a collision and causes further excitation of electron-hole pairs. In other words, there occurs carrier multiplication of carriers in the carrier multiplication layer 13 as a result of the reverse biasing.
Thus, such an avalanche photodiode has a gain and can amplify a photocurrent. Because of this, it becomes possible to reduce the load of a pre-amplifier that is provided behind the photodiode for processing an output signal thereof, by using an avalanche photodiode for optical detection. Because of the foregoing advantageous feature, the avalanche photodiode 10 of FIGS. 1A and 1B is used widely in the optical fiber telecommunication system that transmits optical signals with a transmission rate of 10 Gb.
FIG.2 shows the general construction of an optical fiber telecommunication network that uses the avalanche photodiode 10 for the detection of optical signals.
Referring to FIG. 2, original data 21, which may contain audio and/or video data, is converted into an electrical signal 22, and the electrical signal 22 is used to modulate a laser diode 23 for producing an optical signal. The optical signal thus produced is injected into an optical fiber 24 at a first end and is transmitted therethrough, wherein the optical signal is detected, at the other end of the optical fiber 24, by a photodetector. As a result of photodetection, audio and/or video data 27 corresponding to the original data 21 is reproduced from an output electrical signal 26 of the photodiode 10.
As represented in FIG. 2, the optical fiber 24 has a transmission loss characterized by a minimum loss in the wavelength band (called C-band) between 1530 nm and 1570 nm. Thus, it has been practiced in conventional optical fiber transmission systems to use the foregoing C-band for transmitting optical signals with the conventional transmission rate of 10 Gb/s.
On the other hand, recent widespread use of digital telecommunication as in the case of Internet is causing the problem of sharply increasing signal traffic, and there is emerging a situation in which the use of so-called L-band, which is located between 1570 nm and 1610 nm and has not been used heretofore, is unavoidable.
In the L-band, however, the InGaAs/InP avalanche photodiode, while has been used successfully for the detection of optical signals of the C-band with the transmission rate of 10 Gb/s, cannot provide a satisfactory sensitivity in view of the fact that the fundamental absorption edge of the InGaAs optical absorption layer 12 is located at the wavelength of 1.63 xcexcm (0.76 eV) at room temperature, provided that an InGaAs mixed crystal having a lattice matching composition to the InP substrate 11 is used for the optical absorption layer 12. Thus, in order to continue using the InGaAs/InP avalanche photodiode for the detection of optical signals also in the L-band, it is inevitable to use a high-gain preamplifier for processing the output of the avalanche photodiode.
Further, there is a proposal, in order to deal with the problem of sharp increase of traffic in optical fiber telecommunication systems, to increase the transmission rate of the optical signals from the current rate of 10 Gb/s to 40 Gb/s in the C-band while continuously using the avalanche photodiode of the InGaAs/InP system. This approach, however, raises a problem, associated with the small ionization coefficient ratio of InP, in that it is difficult to reduce the avalanche build-up time in the avalanche photodetector to the degree needed for detecting the optical signal of the transmission rate of 40 Gb/s. It should be noted that the avalanche build-up time is inversely proportional to the ionization coefficient ratio, while the ionization coefficient ratio of InP takes a value of about 1.5 under a practical electric field (6-7xc3x97108V/cm).
Thus, it has been necessary to use a PIN photodiode in combination with an optical amplifier when detecting an optical signal of the C-band and transmitted with the rate of 40 Gb/s. However, such a construction is complex and increases the cost of the optical telecommunication system.
Thus, there has been no practical means for processing the optical signal of the L-band and having the transmission rate of 40 Gb/s.
Accordingly, it is a general object of the present invention to provide a novel and useful photodetector wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a high-speed photodetector having a sufficient sensitivity to optical signals of the L-band by using an optical absorption layer having a large ionization coefficient ratio.
Another object of the present invention is to provide a photodetector, comprising:
a substrate; and
an optical absorption layer provided on said substrate,
said optical absorption layer comprising a mixed crystal of Si, Ge and C.
According to the present invention, the optical absorption layer has a reduced bandgap and the photodetector shows a sufficient photosensitivity against optical signals of the L-band having a longer wavelength. By incorporating C with a concentration of 3% or more in terms of atomic percent, the foregoing mixed crystal forming the optical absorption layer has a direct-transition band structure. By using such a mixed crystal of SiGeC for the optical absorption layer, it is possible to construct any of an avalanche photodiode and a PIN photodiode.
When the SiGeC mixed crystal is used for the optical absorption layer of an avalanche photodiode, it is possible to maximize the response speed by using Si having a large ionization coefficient ratio for a carrier multiplication layer. Thereby, the avalanche photodiode can detect the optical signals not only of the C-band but also of the L-band and having the transmission rate of 40 Gb/s.
Further, it is possible to construct a PIN photodiode by using the SiGeC mixed crystal for the optical absorption layer. In this case, a response speed up to the frequency band of 90 Ghz can be obtained.